For printed circuit boards, it is necessary to print aligned circuit patterns on both sides of an inner core. The inner cores are composed of surface layers of a conductive material, such as copper, on both sides of a dielectric substrate, with the conductive surface layers being selectively removed according to patterns established by lithography to form circuit features. These inner cores will eventually get laminated together to form the final printed circuit board. When a lithography unit exposes each side of the inner core separately, it is important to have a lateral alignment scheme to match position coordinates on both sides so that the front and back patterns will be properly aligned to each other.
The usual alignment method first creates special alignment marks, such as drill holes or patterned features, on one or both sides of the core to establish position references. At least two such marks are needed to account for both lateral x-y displacements in the plane of the surface and rotational skew about any axial point in the plane of the surface. Cameras or physical sensors locate such special marks to determine the position of the work piece.
One example of such an alignment system is found in U.S. Pat. No. 6,701,197 (Ben-Ezra et al.), which writes an alignment pattern on one side of a printed circuit board, while a first lithographic image is written on the other side of the board. The alignment pattern is viewed when the board is flipped over. A second lithographic image is then written on the same side as the alignment pattern. Knowledge of the alignment pattern location defines the position of the already written first image that is now on the underside, so the second image aligns with the first. Other examples of backside alignment systems for lithography that use alignment marks formed on the substrate are found in U.S. Pat. Nos. 6,525,805 (Heinle), 6,861,186 (Pagette et al.), and 6,936,385 (Lof et al.).
It is desirable in many cases to avoid creating any holes in the substrate, so this is a less than adequate alignment solution. Formation of special alignment marks on the substrate is better, but also is not always desired. Additionally, both techniques have accuracy issues. The accuracy of alignment marks is limited by the size of the marks themselves, and finer control of alignment is sought.